The invention relates to a chemiluminescent gas analyzer for determining a concentration of a gaseous component in a sample gas mixture, comprising: a measuring chamber; input conduits for delivering an ozone-containing gas and said gas mixture into said chamber, and at least an outlet for removing said gaseous substances and possible chemical compounds thereof; radiation sensitive detector(s) receiving radiation emitted as a consequence of a reaction between the gaseous component and the ozone-containing gas; an ozonizer for producing said ozone-containing gas. The invention also relates to a method for determining a concentration of a gaseous component in a sample gas mixture, the method comprising the steps: generating an ozone-containing gas; allowing said ozone-containing gas and said gas mixture to stream into a measuring chamber and mix with each other; detecting an intensity of radiation emitted as a consequence of a reaction between the gaseous component and the ozone-containing gas; removing said gases and possible chemical compounds thereof from said chamber, as well as to a method for lowering the quantity of adverse chemiluminescent reactions in a measuring chamber fed by an ozone-containing gas and a sample gas mixture, whereupon a concentration of a gaseous component in said sample gas mixture is to be determined by detecting an intensity of radiation emitted as a consequence of reactions between the gaseous component and the ozone-containing gas.
Nitric oxide (=NO) can be measured in very small concentrations and with short response time using the chemiluminescence technique, whereupon ozone (=O3) or ozone-containing gas is used as a reactant, i.e. here chemiluminescence occurs in a gaseous state. Gas phase chemiluminescent reactions occur also between ethylene and ozone, between carbon monoxide CO and atomic oxygen and between dimethylsulfid and fluorine F2 forming hydrogen fluoride HF in excited state, etc. Since nitric oxide was found to be a signal substance in the body of human beings or animals it has been more and more important to be able to reliably measure nitric oxide entering or exiting the body. Most often nitric oxide is measured in breathing gas either in connection with treatment of lung disease or during asthma diagnostics and as a measure of response to a treatment. The normal requirement for this purpose is a sensitivity of about 1 ppb (parts per billion=10xe2x88x929), and a response time of about 200 ms for a breath-by-breath recording. The concentration of nitric oxide in the exhaust gases from internal combustion engines is also one area in which the chemiluminescence technique is utilized, but in this case a considerably lower sensitivity in the ppm range (parts per million=10xe2x88x926) is sufficient. Typical chemiluminescent gas analyzers are described in U.S. Pat. Nos. 4,822,564 and 6,099,480. As with any luminescence reaction, also in this kind of gas phase chemiluminescence the signal available is quenched by molecules of other gas components in the gas mixture measured. To minimize this harmful quenching of excitation, it is common practice to use very low pressure in the reactor chamber, where NO is reacting with ozone to form nitrogen dioxide in an excited state. The pressures in the reaction chambers are generally suggested to be below 0.01 bar. The chemiluminescent reaction should take place within the reaction chamber so that all created light could be detected. For a sample flow of about 200 ml/min this means that the reaction chamber has to be quite big, often more than 100 cm3, whereupon the light collection has quite a low efficiency. So relatively high NO-concentrations, in the order of tens ppm, can generally be detected using the systems of the above-mentioned publications.
The ozone to be fed for attaining the chemiluminescent reaction is produced using an ozonizer with air as its input gas. The ozonizers are generally based on electrical discharge, preferably on a so called silent discharge, as described in the publication: IEEE Transactions on Plasma Science, Vol.19, No. 2, April 1991 (309-323)-B.
Eliasson and U. Kogelschatz xe2x80x9cModeling and Applications of Silent Discharge Plasmasxe2x80x9d. In such a discharge ozone is generated from oxygen but also different oxides of nitrogen are produced interfering with ozone generation, which is disadvantageous for industrial ozone generation using large installations. According to the patent U.S. Pat. No. 5,792,326 the ozone gas transport path is furnished with means for removing nitrogen oxides using a vessel filled either with an adsorbing zeolite material or dissolving pure water for passage of the gas in the form of tiny bubbles. Hence the object is to purify the ozone gas emerging from the ozonizer so that the ozone gas would be useful for semiconductor fabrication processes and problems caused by contamination with chromium, originating from stainless steel components, could be avoided. Because the ozone gas is here use for semiconductor fabrication processes the concentration of ozone should also be high. The purifying materials, like zeolite and water, require regular service and material changing at certain intervals. Additionally there are considerable material expenses.
It is known that ozone is a very reactive gas and so it can react with a number of different materials and constituents of materials and various other chemicals present. For example dirt and organic materials originating from a patient together with the sample gas often cause some chemiluminescent reactions, which is detected as a high dark level or background signal in the analyzer. The high dark level or background signal cannot be distinguished from the correct radiation mathematically, and so conducts to receiving erroneous concentration values. In the chemiluminescent gas analyzers, the light or radiation caused by this kind of unintended chemiluminescent reactions and/or the fluorescent radiation interfering with wavelengths utilized for the actual measuring of concentration is/are eliminated, if necessary, using an optical filter or optical filters between the measuring chamber and the detector or detectors, as shown in the non-published patent application EP-01660055.3 of the applicant. The optical filter(s) can be long pass filter(s), short pass filter(s), or band pass filter(s). E.g. in case nitric oxide NO reacting with ozone O3 is measured, an optical long pass filter having transparence over about 620 nm is often used.
The object of the present invention is to improve the elimination of that possibly emitted weak light or radiation not proportional to the concentration of the gas component(s), which is/are measured utilizing ozone based chemiluminescent reactions in a measuring chamber. This means that the adverse effects of light or radiation caused by at least other than intended chemiluminescent reactions should be minimized. Another object of the present invention is to attain a small sized chemiluminescent analyzer with high sensitivity in respect to those wavelengths, which are excited by that or those chemiluminescent reaction(s) depending on the gaseous components to be measured in the sample gas mixture. Further an object of the present invention is to attain a chemiluminescent analyzer, which does not consume material and require frequent service, if only possible, at least for the purpose of lowering or eliminating high dark level or background signal.
The above-defined objects can be achieved by means of a chemiluminescent gas analyzer according to the invention comprising a flow delay device in the input conduit for said ozone-containing gas and downstream from the ozonizer, and said flow delay device having a delay volume causing a predetermined delay time for the flow of the ozone-containing gas from said ozonizer to said measuring chamber. Further, the above-defined objects can be achieved by means of a method for determining a concentration of a gaseous component in a sample gas mixture by delaying the flow of said ozone-containing gas between the generation thereof and said streaming into the measuring chamber with a predetermined delay time, or by means of a method for lowering the quantity of adverse chemiluminescent reactions, in which the flow of the ozone-containing gas is delayed so as to provide a predetermined delay time after the generation of the ozone in said ozone-containing gas and prior said reactions within the measuring chamber.
Now it has surprisingly noticed that a considerable portion of the high dark level or background signal is caused by the ozone-containing gas alone when entering the measuring chamber, at least in small sized analyzers, in the absence of any sample gas and any contamination. The possibility to this kind of light emission is confirmed by the publication: Gmelins Handbuch der Anorganischen Chemie, Stickstoff, Vol. 4, 1936 p. 628, which describes that the decomposition of NO in an ozonizer goes through three intermediate steps, whereupon the color of the emitted light changes from yellow at the first step, gradually through yellow-red and fiery red to blue-magenta at the end. Further it has surprisingly been noticed that the adverse effect of these spontaneous reactions on the measuring accuracy and reliability can be avoided by simply delaying the flow in the ozone line after the ozone generation so that the light producing reactions have time to fade out. The improvement according to the invention is to provide the delay time accomplished using a specific volume depending on the flow rate of the ozone-containing gas. As a result, the dark signal decreases to the same level as if no nitrogen oxides were present at all. The delaying volume can be in almost any form and material providing that no adsorption of the ozone occurs and that is resistive to the gases. So, according to the invention there is no need for trying to remove the nitrogen oxides, because they do not any more disturb the chemiluminescent measurement even in a sensitive analyzer. Further, there is no need for service concerning changing of material(s), since no part of the delay device is consumed.
The invention is now described in detail with reference made to the accompanying drawings.